An emulsion is a dispersed system containing at least two immiscible liquid phases, one of which is dispersed in the form of small droplets throughout the other, and an emulsifying agent in order to improve the stability of the system.
There are two types of emulsions depending on the droplet size of the liquids present in the emulsions. Macroemulsions do not permit light to pass through them since the droplets have average diameters of about 10 to about 1000 microns. These emulsions typically appear milky white. Microemulsions are stable systems consisting of droplets which are significantly smaller, being approximately 0.2 microns, or smaller, in diameter on the average. As such, microemulsions are translucent, and routinely transparent, in appearance.
Microemulsions are an extraordinary type of emulsion that forms spontaneously. Products consisting of these systems are valued for their stability and small particle size, which affords microemulsions special consideration in the market place. Additional information on microemulsions and their properties may be obtained from Emulsions and Emulsion Technology Part I, ed. Kenneth J. Lissant, Marcel Dekker, Inc., New York, Chapter 3 "Microemulsions"(1974) and Microemulsions Theory and Practice ed. Leon M. Prince, Academic Press, Inc. (1977).
Emulsions have a variety of uses, most notably as vehicles for the delivery of medicines. In particular, microemulsions are known to deliver pharmacologically active agents as disclosed in U.K. Patent No. 2,098,865. Microemulsions are also known to provide injectable compositions containing an anaesthetic, as disclosed in U.S. Pat. No. 3,917,830, and are known as carriers for oxygen absorbing fluorinated organic compounds, as disclosed in U.S. Pat. No. 3,778,381. Jayakrishnan et al. in J. Soc. Cosmet. Chem. 34, 335-350 (1983) disclose the delivery of hydrocortisone with microemulsions. Finally, U.S. Pat. No. 4,146,499 discloses oil-in-water microemulsions allegedly capable of containing a variety of hydrophobic substances including certain cosmetics.